1. Field of the Invention
The present invention provides a multilayer coating excellent in wear resistance and heat resistance and, more specifically, a multilayer coating including an alumina coating formed on a hard coating to be applied to a wear resistant member such as a cutting tool, a sliding member or a die. The multilayer coating is excellent in the adhesion between the alumina coating and the hard coating and can keep satisfactory wear resistance and heat resistance over a long period while preventing peeling of the alumina coating from the hard coating.
Although an alumina coating obtained by the present invention is applicable to various purposes as described above, its application to a cutting tool will be mainly illustrated hereinafter as a typical example.
2. Description of the Related Art
In a cutting tool or sliding member which requires excellent wear resistance or sliding characteristic, it has been generally adapted to ensure the heat resistance by forming a hard coating of titanium nitride, titanium aluminum nitride or the like on a base material surface of high-speed steel, cemented carbide or the like by means of chemical vapor deposition (hereinafter referred to as CVD method), and further forming an aluminum oxide (particularly, α-alumina having corundum structure) thereon.
However, since heating of the base material to 1000° C. or higher is required, particularly, when the α-alumina having corundum structure is formed by the CVD method, applicable base materials therefor are limited. A certain kind of base materials softens, when exposed to a high temperature of 1000° C. or higher, and loses the suitability as a base material for wear resisting member. Even a high-temperature base material such as cemented carbide causes a problem such as deformation, when exposed to such a high temperature.
In order to solve the above problems, a process using physical vapor deposition (hereinafter referred to as PVD method) capable of forming α-alumina at a relatively low temperature has been proposed. For example, Japanese Patent Laid-Open No. 2002-53946 discloses a process for forming, by use of an oxide coating of corundum structure (α-type crystal structure) with a lattice parameter of 4.79 Å or more and 5.000 Å or less and a film thickness of at least 0.005 μm as an under layer, an alumina coating of α-type crystal structure on this under layer. According to this process, the component of the oxide coating preferably consists of any one of Cr2O3, (Fe,Cr)2O3, and (Al,Cr)2O3, (Fex,Cr(1−x))2O3 (x: 0≦x≦0.54) is more preferably adapted when the component of the oxide coating is (Fe,Cr)2O3, and (Aly,Cr(1−y))2O3 (y: 0≦y≦0.90) is more preferably adapted when the component of the oxide coating is (Al,Cr)2O3.
It is also denoted in Japanese Patent Laid-Open No. 2002-53946 that crystalline α-alumina can be formed even at a relatively low base material temperature by adapting a process for forming a composite nitride coating of one or more elements selected from the group consisting of Ti, Cr and V with Al as a hard coating, forming a coating composed of (Alz,Cr(1−z))N (z: 0≦z≦0.90) as an intermediate layer, oxidizing this coating to form an oxide coating of corundum structure (α-type crystal structure), and forming α-type alumina on the oxide coating.
The present inventors also have proposed, in order to obtain a coating excellent in heat resistance and wear resistance, to form a TiAlN coating on the surface of a cutting tool or the like as a hard coating and further form an alumina coating mainly composed of α-type crystal structure on this coating (WO2004/015170). Specifically, the process comprises the steps of forming the hard coating (TiAlN coating) on a base material, oxidizing the surface of the hard coating to form an oxide-containing layer, and forming the alumina coating mainly composed of α-type crystal structure on the oxide-containing layer.
According to the process described in WO2004/015170, the productivity is improved, compared with a process comprising the steps of forming a CrN coating after forming a TiAlN coating, oxidizing the CrN coating to form Cr2O3 of corundum structure, and then forming the alumina coating mainly composed of α-type crystal structure thereon as in Japanese Patent Laid-Open No. 2002-53946. Further, the deterioration of cutting performance by a Cr-containing coating such as Cr2O3 layer or (CrN+Cr2O3) composite layer to be formed as the intermediate coating can be also avoided.
However, this method retains the potential to cause the peeling of the hard coating (TiAlN coating) from the base material similarly to the case of Japanese Patent Laid-Open No. 2002-53946, since the base material temperature needs to be raised to about 700 to 750° C. in an oxidizing atmosphere when the alumina mainly composed of α-type crystal structure is formed on the oxide-containing layer after oxidizing the surface of the hard coating (TiAlN coating).
In view of the above-mentioned problem, the present inventors have also proposed to keep the adhesion between the base material and the hard coating by preliminarily forming, in case of forming a hard coating composed of a compound of a metallic component containing Al and Ti as essential components with C, N, B, O, etc., at least one layer selected from the group consisting of metallic layers and alloy layers to be oxidized at a temperature lower than 700° C., which is lower than the oxidizing temperature of the hard coating, and compound layers of such metals or alloys with C, N, B, O, etc. on the base material as an under layer.
However, even if such a multilayer coating is formed, deterioration of adhesion may be caused between the hard coating and the alumina coating in a remarkably severe use environment, and excellent wear resistance and heat resistance cannot be sufficiently maintained.